An improved alarm system for monitoring movement through a passageway defined by opposing sidewalls. The alarm system has a lower passage indicator disposed toward the lower ends of the sidewalls, an upper passage indicator disposed toward the upper ends of the sidewalls and a control mechanism configured to sound an alarm if the lower passage indicator indicates passage through the passageway and the upper passage indicator does not. In one embodiment, each of the passage indicator comprises transducers that are configured to direct and receive sound waves, such as ultrasound waves, and provide lower and upper ultrasound waves that act as lower and upper barriers. If only the lower barrier is interrupted, indicating a child passing through the passageway, the control mechanism will generate an alarm signal. If both barriers are interrupted, indicating an adult passing through the passageway, the control mechanism will not generate the alarm signal.